Madagascar hosts one of the world’s most unusual, endemic, diverse and threatened concentrations of fauna1. To explain its unique, imbalanced biological diversity, G. G. Simpson proposed the ‘sweepstakes hypothesis’, according to which the ancestors of Madagascar’s present-day mammal stock rafted there from Africa2. This is an important hypothesis in biogeography and evolutionary theory for how animals colonize new frontiers1,3,4,5, but its validity is questioned5,6,7,8,9. Studies suggest that currents were inconsistent with rafting to Madagascar9 and that land bridges provided the migrants’ passage5,6,7,8. Here we show that currents could have transported the animals to the island and highlight evidence inconsistent with the land-bridge hypothesis. Using palaeogeographic reconstructions and palaeo-oceanographic modelling, we find that strong surface currents flowed from northeast Mozambique and Tanzania eastward towards Madagascar during the Palaeogene period, exactly as required by the ‘sweepstakes process’. Subsequently, Madagascar advanced north towards the equatorial gyre and the regional current system evolved into its modern configuration with flows westward10 from Madagascar to Africa. This may explain why no fully non-aquatic land mammals have colonized Madagascar since the arrival of the rodents and carnivorans during the early-Miocene epoch. One implication is that rafting may be the dominant means of overseas dispersal in the Cenozoic era when palaeocurrent directions are properly considered.
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M. Nowak, W. de Ruijter, I. Tattersall and A. Yoder supplied reprints. J. Aitchison, R. Corlett and A. Switzer are thanked for sharing information. M.H. is supported by US National Science Foundation (NSF) grant 0927946-ATM and uses the US National Center for Atmospheric Research CCSM, which is supported by the NSF. M.H. acknowledges conversations with P. Koch and D. Raup on vicariance biogeography. All computing was performed at the Rosen Center for Advanced Computing, which is part of Information Technology at Purdue, Purdue University.
Author Contributions J.R.A. initiated the study and was primarily responsible for the geologically related aspects. M.H. carried out the palaeo-oceanographic modelling and its interpretation. Both authors contributed to the writing of the paper.
The authors declare no competing financial interests.
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Ali, J., Huber, M. Mammalian biodiversity on Madagascar controlled by ocean currents. Nature 463, 653–656 (2010). https://doi.org/10.1038/nature08706
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